Power semiconductor components, which have a drift zone and a field electrode arranged adjacent to the drift zone and isolated from the drift zone by a dielectric, are known in principle and described for example in U.S. Pat. No. 4,903,189 (Ngo), U.S. Pat. No. 4,941,026 (Temple), U.S. Pat. No. 6,555,873 B2 (Disney), U.S. Pat. No. 6,717,230 B2 (Kocon), U.S. Pat. No. 6,853,033 B2 (Liang).
The provision of a field electrode which is insulated from the drift zone and which is at the potential of the source terminal in the case of a MOSFET, for example, enables a higher doping of the drift zone and thus leads to a reduction of the “on” resistance of the component for the same dielectric strength. In the case of these components, the field electrode provides an opposite charge with respect to the charge that is present in the drift zone and results from the doping. Charge carriers in the drift zone are compensated for by this opposite charge, so that the dielectric strength of the component is not reduced despite higher doping of the drift zone.
In order to afford a better understanding of the invention explained below, the basic principle of such a component will be explained first with reference to a power MOSFET illustrated in FIG. 1.
The MOSFET of FIG. 1 has a drift zone 12 arranged in a semiconductor body 100, said drift zone being arranged between a drain zone 11 and a body zone 13 in a vertical direction of the semiconductor body 100. A field electrode 23, sometimes in the form of a field plate, is arranged adjacent to the drift zone 12 in a lateral direction. The field electrode 23 is insulated from the drift zone 12 dielectrically by a dielectric layer 24 and may extend over the entire depth of the drift zone 12 in the vertical direction. At two or more sides, depending on the geometry of the drift zone, the drift zone 12 may additionally adjoin a plurality of field electrodes 23 that are at a common potential.
The body zone 13 is arranged between the drift zone 12 and a source zone 15, with which contact is made by a source electrode 31. In this case, the body zone 13 is formed complementarily with respect to the source zone 15, the drift zone 12 and the drain zone 11 and is p-doped in the case of an n-conducting MOSFET and n-doped in the case of a p-conducting MOSFET. The drift zone 12 is doped more weakly than the source zone 15 and the drain zone 11.
In order to form an electrically conductive channel in the body zone 13 between the source zone 15 and the drift zone 12, a gate electrode 21 is present, which is arranged adjacent to the body zone 13 and is insulated from the body zone 13 dielectrically by a gate dielectric 22. In this case, the gate electrode 21 and the field electrode 23 may be arranged in a manner lying one above the other in the vertical direction in a common trench of the semiconductor body 100.
The explanations below relate to an n-conducting MOSFET having n-doped drain, drift and source zones 11, 12, 15 and a p-doped body zone 13. Such a MOSFET turns on when a positive voltage is applied between the drain zone 11 and the source zone 15 if a suitable positive driving potential is present at the gate electrode 21. The component turns off in the case of a positive drain-source voltage in the absence of a suitable driving potential at the gate electrode 21 for forming a conductive channel in the body zone 13. The pn junction between the body zone 13 and the drift zone 12 is reverse-biased in this case. Proceeding from said pn junction, a space charge zone propagates in the drift zone 12 in the direction of the drain zone 11. The extent of said space charge zone, which is also referred to as depletion zone, is dependent on the reverse voltage present. Within the space charge zone, positively charged donor cores are present in the drift zone 12 and, in the absence of a field electrode, they find their opposite charge in the form of acceptors, to which an electron is bonded, exclusively in the body zone 13. The formation of said space charge zone leads to an electric field which has its maximum field strength in the region of the pn junction and the field strength of which decreases in the drift zone 12 in the direction of the drain zone 11. In this case, the field strength may rise maximally up to a critical field strength dependent on the respective semiconductor material before an avalanche breakdown occurs. Said critical field strength Ekrit is, in the case of silicon, for example, approximately Ekrit=400 kV/cm for a doping concentration of 1016 dopant atoms/cm3. In this case, the maximum field strength of the electric field increases, for the same reverse voltage present, with the number of positively charged donor cores which find their opposite charge in the body zone 13.
The functional principle of the MOSFET having a field plate 23 as illustrated in FIG. 1 is based, then, on making available opposite charges for positively charged donor cores of the drift zone 12 in the field electrode 23 adjoining the drift zone 12 in the lateral direction. As a result, this has the effect that the space charge zone can propagate further for a given doping level in the vertical direction of the drift zone before the critical field strength is reached, which ultimately leads to an increase in the breakdown voltage. This is equivalent to being able to implement a higher basic doping of the drift zone 12 for a predetermined breakdown voltage, which in turn leads to a reduction of the on resistance of the component.
The proportion of positively charged donor cores in the drift zone 12 which can be compensated for by means of the field electrodes is limited by the so-called breakdown charge QBR of the semiconductor material used, for which the following holds true:QBR∈0·∈r·Ekrit/q  (1)
Said breakdown charge is dependent only on the dielectric constant of the semiconductor material used and the elementary charge q. A breakdown charge of 2.6·1012/cm2 results for the value of the abovementioned critical field strength Ekrit =400 kV/cm of silicon. For lower doping concentrations, this value of the breakdown charge decreases depending on the critical field strength that is lower in these cases in accordance withEkrit=4.01 kV/cm·(ND·cm3)1/8  (2)wherein ND denotes the doping concentration.
The breakdown charge specifies how many positively charged donor cores per unit area are permitted to be present which find an opposite charge in one direction before an avalanche breakdown of the semiconductor material used occurs.
The compensation of charge carriers in the drift zone 12 by means of the field electrode 23 presupposes a capacitive coupling between the drift zone 12 and the field electrode 23 by means of the dielectric layer 24 and also a voltage difference between the drift zone 12 and the field electrode 23. What is problematic in this case is that a maximum compensation effect occurs only in those regions of the drift zone 12 which are at a distance from the pn junction in a vertical direction since it is only in these regions that there is a sufficiently high voltage drop between the drift zone 12 and the field electrode 23 to compensate for a charge corresponding to the breakdown charge in the drift zone 12. The capacitance—referred to hereinafter as “coupling capacitance”—between the field electrode 23 and the drift zone 12 could be increased by reducing the thickness of the dielectric layer 24. However, this would make the dielectric layer sensitive to voltage spikes that can occur during operation of a power semiconductor component.
FIG. 2 illustrates as a dashed line the profile of the electric field in the drift zone 12 in the vertical direction proceeding from the pn junction, assuming that a reverse voltage is present at which the electric field strength in the region of the pn junction lies just below the value of the critical field strength Ekrit. Proceeding from the pn junction, the position of which corresponds to point x0 of the system of coordinates in FIG. 2, the field strength initially decreases rapidly in the direction of the drain zone 11. The reason for this is that in the region adjoining the pn junction there is not yet a compensation effect by means of the field electrodes 23, so that positively charged donor cores find their opposite charge exclusively in the body zone 13. The profile of the electric field strength E flattens out starting from a position x1, from which a voltage drop between the drift zone 12 and the field electrode 23 is high enough to compensate for a charge corresponding to the breakdown charge. Starting from a point x2 indicating the junction between the weakly doped drift zone 12 and the highly doped drain zone 11, the electric field decreases rapidly to zero owing to the high doping concentration in the drain zone 11.
FIG. 2 illustrates as a dash-dotted line the profile of the electric field strength that would result without a field electrode 23.
The breakdown voltage of the component corresponds to the integral of the electric field strength over the length d of the drift zone. As can be seen, the compensation by the field electrodes 23 leads to a significant gain in breakdown voltage given the same doping of the drift zone 12. The breakdown voltage does not reach its maximum value, however, on account of the lack of compensation of the charge carriers in the region adjoining the pn junction.